I believe on this list the only one that qualifies is Mac Callander. The protagonist needs to be a Gunslinger, so Pearson is not eligible. I'm pretty sure with all the mystery around the Blackwater Massacre, we eventually going to relive the action on the ferry. Lenny and Kieran were not on the ferry. Sean MacGuire was knocked out early when things turned wrong. So the only who actively took part in the gunfight on the ferry is Mac Callander.
Mac could still technically be alive, compare to Davey, Lenny and Sean, he is the only one that his death was not witness by any member of the gang, and he also one of the only character with no grave. If they wants him to be still alive, they could just pretend that the Pinkerton agent lied to Arthur so the Gang would not attempt to free the man like they did with Sean.
And story from uncle of Mac beating 15 sailors in a brawl, or Hosea stories of the night in Deer Creek sounds like potential mission for RDR3.